1. Field of the Invention
The present invention relates to a wafer processing apparatus for processing a wafer, such as a semiconductor wafer, in a vacuum chamber and, more particularly, to a wafer processing apparatus suitable for use as a plasma processing apparatus which needs to cool or heat the wafer during plasma processing.
2. Description of the Related Art
Formation of a minute pattern when fabricating solid state microelements for a semiconductor integrated circuit is one of the important uses of a plasma processing apparatus, such as a dry etching apparatus using microwave discharge. When processing a semiconductor wafer by a dry etching process, the compatibility of anisotropy and selectivity is an important characteristic of the dry etching process. Various efforts have been made to realize a dry etching process capable of securing the compatibility of anisotropy and selectivity. A low-temperature etching technique using an ion-assisted reaction is essential to forming a submicron pattern.
When forming a submicron pattern on a wafer, the wafer is heated by heat generated by the chemical reaction of the wafer and plasma, and the energy of incident ions and electrons and, consequently, the temperature of the wafer rises. To suppress the rise of the temperature of the wafer, a wafer holder holding the wafer is cooled with a cooling medium, such as water, methanol or liquid nitrogen, the wafer is held in close contact with the surface of the wafer holder by electrostatic attraction to transfer heat efficiently from the wafer to the wafer holder, and the space between the wafer and the wafer holder is filled with a heat transferring gas, such as He gas, of a pressure in the range of about 1 torr to about 10 torr to transfer heat efficiently from the wafer to the wafer holder for efficient cooling of the wafer.
Alternatively, in some applications, it is preferable to provide a heating gas. For ease of discussion, the following description will be directed to cooling, noting, however, that the present invention can be directed to systems using heating gas as well.
A vacuum processing apparatus provided with such means for cooling the wafer is disclosed, for example, in U.S. Pat. No. 4,771,730. This known vacuum processing apparatus supplies a cooling gas, such as N.sub.2 gas, Ar gas or He gas, into the minute space between the surface of a wafer holder and the backside of a wafer so that heat is transferred efficiently from the wafer to the wafer holder, which is cooled sufficiently, to cool the wafer. This vacuum processing apparatus employs electrostatic attraction to hold the wafer in close contact with the wafer holder and is not provided with any wafer clamping mechanism. The omission of the wafer clamping mechanism prevents insulation failure and the like attributable to fine particles chipped off from the component parts of the wafer clamping mechanism during plasma processing and adhering to the wafer.
However, although electrostatic attraction is effective while the plasma processing apparatus is in operation, the electrostatic attraction disappears when the plasma disappears due to the instantaneous failure of the main power source, the failure of the power source for the microwave generator or the failure of the RF power source. Since the cooling gas is supplied into the space between the surface of the wafer holder and the backside of the wafer, the wafer is caused to float by the pressure of the cooling gas, the wafer is dislocated and, if things come to the worst, the wafer will fall off the wafer holder. A method employing a wafer holder provided with a dielectric film having a large dielectric constant to increase the residual electric charge remaining after the wafer holder has been disconnected from the power source may be able to enhance the residual electrostatic attraction. However, this methods increases the time necessary for removing electrostatic charge in the normal operation of the apparatus, which increases the wafer processing cycle time and reduces the throughput of the apparatus. Accordingly, this method is not practical.